For supplying power to electric operating equipment, the network configuration of an ungrounded (IT) power supply system (from French isolé terre) is used when it comes to higher requirements in operational, fire and contact safety. In this type of power supply system, the active parts of the electric installation are separated from the ground potential.
Via the inherent safety of the ungrounded power supply system, a perpetual power supply of the appliances supplied by the ungrounded power supply system, i.e. the operating equipment connected to the ungrounded power supply system, can thus be ensured even if a first insulation fault occurs.
This presupposes that in the network to be monitored the resistance from active conductors of the power supply system to ground (insulation resistance—also called insulation fault resistance or fault resistance in the event of a fault) will have to be constantly monitored since a fault loop could arise via a possible further fault at another active conductor (second fault) and the flowing fault current in conjunction with an overcurrent protective device would result in the installation having to be shut down, leading to an operational standstill.
For monitoring the insulation of ungrounded power supply systems, insulation monitoring devices (IMDs) are used whose requirements are defined in the international standard IEC 61557-8.
However, there are applications, in particular in conjunction with using converters in the megawatt power range, in which high fault currents can flow via paths upon a first fault occurring, these paths not being designed for such high currents. This can result in operating states dangerous to persons and the installation so that the power supply is to be automatically shut down in the first fault instance. The power supply system is shut down by the insulation monitoring device by means of a shut-down signal sent to the alarm output of the insulation monitoring device.
This examination shows that higher demands to reliability are to be made to the usage of an insulation monitoring device regarding the modes of application described above. Similar to known protective devices, such as residual current protective devices (RCDs), a periodic monitoring of the function of the insulation monitoring device is required for ensuring that the installation is shut down reliably in the event of a fault.
Presently, a periodic monitoring of the insulation monitoring device is often only carried out within the scope of a mandatory repeat test of the power supply system, mostly partly automated using an installation testing device or a (self-made) testing apparatus, such as test fault resistances added via a relay.
Installation testing apparatuses known from the state of the art, e.g. from the company Benning temporarily add a fault resistance from an active conductor to ground and log the behavior of the insulation monitoring device to be tested. However, the testing operation must be triggered via manual entries and be monitored by the user. Moreover, the installation testing apparatuses for the partly automated testing of insulation monitoring devices are generally unsuitable for power supply systems ranging over a nominal voltage of 400 V and do not make enough provisions for the specific application.
When the insulation monitoring device is tested solely manually, the user either adds a suitable fault resistance to the energized power supply system from an active conductor to ground or the installation has to be repeatedly shut down and rebooted during testing in order to be able to add and remove the fault resistances without any danger. These modes of procedure are either very dangerous for personnel or so time-consuming when being executed that a test remains undone.
Simple, partly automated testing apparatuses used in practice, either made by the installation operator or acquired externally, pose another threat. These testing apparatuses generally consist of the following elements: test resistance, electromechanical switch (relay, contactor) and a switch or a push-button.
Should the test fault resistance be added via a test button, this button must be pressed down over a sufficient amount of time depending on the installation, the operational state and the insulation monitoring device put to use. In consequence, it is often preferred to provide a switch instead of a button, said switch maintaining its flipped position and adding the test fault resistance to ground from the active conductor as long as the switch is not flipped back. These solutions are potentially dangerous since there is a risk of the switch not being flipped again or at least not on time for removing the test fault resistance. The power supply system would then be operated too long or even constantly using a first fault. Should the used testing apparatus not be constructed for the overvoltage occurring in this process, damage to persons and property can arise due to fire hazards during this constant testing operation.
There is no product standard for partly automated, permanently installed testing apparatuses for testing the function of the insulation monitoring devices, which is why the installation operator has to assess the risks of each solution under their own responsibility.
When using a self-made or externally acquired, partly automated and permanently installed testing apparatus—which still requires manual inputs—, testing many different operating states of the installation only becomes possible with great effort in order to be able to provide well-founded information on the reliable function of the insulation monitoring device.
In most applications, testing the function of the insulation monitoring device by externally inserting a fault resistance in the installation is omitted.
Another disadvantage when using installation testing apparatuses as well as when conducting a solely manual test is that only exactly one current operational state of the power supply system to be monitored is tested during the testing period. Information regarding the monitoring reliability in other system states can only be provided when employing a much greater effort during testing.
As an alternative measure to this, the functionality of the insulation monitoring device is tested by means of a self-test by manually actuating the test button on the insulation monitoring device. Malfunctions of functional groups in the insulation monitoring device are detected via this internal testing in the device. Some devices can also be set up to actuate the alarm relays within the scope of the manually started self-test in order to establish their switching functionality.
A part of the tests conducted during the manual self-test are automatically repeated in some insulation monitoring testing devices, e.g. every 24 hours. Separate alarm outputs for device faults are usually available in these devices.
The installed self-test function, however, cannot test whether interference voltage components existing between the monitored ungrounded power supply system and ground are systemically interfering with the insulation monitoring that strongly so that the task of monitoring insulations cannot be conducted sufficiently reliably.
It can be said, therefore, that there is no economically feasible solution for monitoring whether an insulation monitoring device sufficiently fulfills the monitoring function during all operational states with respect to the specific application. There is a risk, in particular regarding a cost pressure arising from the application, of insulation monitoring devices being put to use which comprise considerable monitoring gaps.
The object of the present invention is therefore to propose a method and a testing device for testing the function of a standard insulation monitoring device installed in an ungrounded power supply system, the operational requirements of the insulation monitoring device related to the application being automatically considered so that providing well-founded information in regard of the reliability of the insulation monitoring device's function becomes possible.